Epidemiological and individual case studies indicate that genetic factors play a significant role in the genesis of Alzheimer Disease (AD). To date, molecular genetic studies in families multiply affected with AD have identified three genes (Presenilin 1-PS1, Presenilin 2-PS2, and beta-amyloid precursor protein--betaAPP) associated with highly penetrant early onset AD, and one gene (Apolipoprotein E) associated with late onset AD. A fifth potential AD susceptibility locus has been mapped to a broad region of chromosome 12, but the responsible gene defect has not yet been identified. Case-control studies comparing the frequency of alleles in numerous other candidate genes have identified a number of additional potential AD genes. However, methodological difficulties and conflicting results in follow-up studies, make it unclear whether allelic variations in these genes are truly pathogenic. Nevertheless, analysis of the biochemical effects of mutations in PS1, PS2, betaAPP at least, suggest a common biochemical effect-namely disturbances in the processing of betaAPP protein. In addition to utility in defining potential therapeutic targets, in some circumstances these genes can also potentially be used as adjunctives in clinical presymptomatic, symptomatic or pharmacogenomic diagnosis.